Stock-Based Compensation, Option Plans (Details) - Stock Options - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Stock-Based Compensation
Total intrinsic value of stock options exercised (in dollars)	$ 28.9	$ 45.2
Total fair value of stock options vested (in dollars)	0.4	$ 0.6
Total compensation cost related to non-vested options not yet recognized (in dollars)	$ 165.2
Weighted average expected amortization period	3 years 3 months

Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Amount of cost to be recognized for option under share-based payment arrangement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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- Definition

Fair value of options vested. Excludes equity instruments other than options, for example, but not limited to, share units, stock appreciation rights, restricted stock.

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